Microlithography is used for producing microstructured components, such as for example integrated circuits or LCDs. The microlithography process is carried out in a so-called projection exposure apparatus having an illumination device and a projection lens. The image of a mask (reticle) illuminated with the illumination device is in this case projected by the projection lens onto a substrate (for example a silicon wafer) coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection lens, in order to transfer the mask structure to the light-sensitive coating of the substrate.
In projection lenses designed for the extreme ultraviolet (EUV) range, i.e. at wavelengths of e.g. approximately 13 nm or approximately 7 nm, owing to the lack of availability of suitable light-transmissive refractive materials, mirrors are used as optical components for the imaging process. Such EUV mirrors have a mirror substrate and a reflection layer stack—constructed from a multiplicity of layer packets—for reflecting the electromagnetic radiation that is incident on the optically effective surface.
In order to avoid damage to the chemically reactive layer materials of the reflection layer stack by way of EUV radiation incident during operation, it is known, inter alia, to apply a metallic diffusion barrier layer onto the reflection layer stack, which metallic diffusion barrier layer can be produced from e.g. Ru (ruthenium), rhodium (Rh), platinum (Pt), silver (Ag) or iridium (Ir) and is sufficiently tight to suppress a diffusion of e.g. oxygen (O2) from the adjacent gaseous phase into the reflection layer stack.
In practice, however, this involves the problem that such a metallic diffusion barrier layer configured to ensure the highest possible reflectivity or lowest possible absorption with a comparatively small layer thickness (of e.g. 1 nm to 2 nm) can mechanically deform under EUV irradiation to the extent that the metallic diffusion barrier layer in question converges in a substantially spherical manner and in the process becomes detached from the adjoining reflection layer stack. This effect, also referred to as “dewetting”, is shown merely schematically in FIGS. 3A-3B. In these figures, “32” denotes the reflection layer stack and “33” denotes the diffusion barrier layer, the diffusion barrier layer 33 as shown in FIG. 3B converging upon EUV irradiation to form substantially spherical segment-shaped regions 33a-33d. 
The “dewetting” described above has the effect, in turn, that the regions of the reflection layer stack 32 which are affected respectively by the detachment are no longer protected by the diffusion barrier layer 33 from a chemical reaction with the surrounding atmosphere (e.g. oxygen atoms), and a chemical reaction of the reflection layer stack 32 and also an associated significant impairment of the reflection properties of the mirror can take place. Thus, it would be possible to observe a relative loss of reflection on the order of magnitude of 20% in experiments, for example, even given a comparatively low power density of 20 mW/mm2 after an irradiation duration of 200 h.
The problem described above proves to be particularly serious in a typically used reducing (e.g. hydrogen) atmosphere, since in this case the mobility of the atoms located at the surface of the metallic diffusion barrier layer, and thus also the tendency toward “dewetting”, is particularly great.
In relation to the prior art, reference is made merely by way of example to DE 10 2009 043 824 A1, DE 10 2011 083 461 A1, JP 2006170916 A, DE 10 2004 062 289 A1, DE 102 35 255 A1, US 2004/0105145 A1 and EP 2 509 102 A1.